Hydraulic actuators are often used as positioning means for various devices and apparatuses. An example of hydraulic actuator is disclosed in Japanese Patent Laid-Open No. 303,247/1988, where an automotive automatic transmission uses the hydraulic actuator to select a desired gear ratio. In this known construction, mechanical positional output from the actuator is transmitted to the shift lever drive device via a rod and other components to select a desired gear.
Such a positioning hydraulic actuator is shown in FIG. 5 and generally comprises a differential hydraulic cylinder 1 and a hydraulic circuit 3 supplying hydraulic pressure from a hydraulic pressure generator 2 to the cylinder 1. The hydraulic cylinder 1 forms a pressure-receiving chamber 1b having a smaller pressure-receiving area and another pressure-receiving chamber 1a having a larger pressure-receiving area. The chambers 1a and 1b are located on the opposite sides of a piston 10. A normally closed solenoid valve V.sub.1 is connected with the chamber 1b. A normally open solenoid valve V.sub.2 is located between the chambers 1b and 1a. A normally open solenoid valve V.sub.3 is disposed between the chamber 1a and a tank port T. These three valves V.sub.1, V.sub.2, V.sub.3 are controlled by electrical signals. The speed of the cylinder is controlled by energizing and deenergizing the intermediate valve V.sub.2 or controlling the duty cycle of the pulses applied to this valve.
When the piston rod 11 is moved to the right as indicated by the arrow in FIG. 5, the hydraulic actuator has been heretofore controlled in the manner described now. When the piston 10 is started, the normally closed solenoid valve V.sub.1 and the normally open solenoid valve V.sub.3 are simultaneously actuated, as shown in FIG. 6, to shut off the fluid passage from the tank T. The oil under pressure from the hydraulic pressure generator 2 flows into the pressure-receiving chambers 1a and 1b on the opposite sides of the piston 10 through the port in the opened valve V.sub.1 and through the port in the normally open valve V.sub.2. The difference in pressure between the two chambers initiates the operation of the piston 10.
When a given stop start position located ahead of an intended stop position is reached, the intermediate, normally open valve V.sub.2 is actuated to increase the pressure in the chamber 1b. This decelerates the movement of the piston 10. When the piston rod 11 reaches the intended stop position and the operation is complete, all the solenoid valves V.sub.1, V.sub.2, and V.sub.3 are deenergized at the same time to shut off the oil passage.
When the hydraulic actuator is controlled in this way, it is inevitable that considerable large pressure surges are produced in the circuit because of the difference in characteristic between the normally open valve and the normally closed valve. Specifically, when the valve body of a normally closed solenoid valve is shifted out of its closed position, if the port or passage opens at all, the oil under pressure is released and gushes out. Therefore, the normally closed solenoid valve responds very quickly. On the other hand, a normally open solenoid valve is closed after it is open, the valve body closes the port so as to obstruct the flow of the circulating oil under pressure. Therefore, it takes long to completely close the port after the valve is started to be closed, i.e., the valve responds slowly.
For this reason, if the normally closed solenoid valve V.sub.1 and the normally open solenoid valve V.sub.3 are simultaneously actuated in a straightforward manner at the beginning of the operation of the hydraulic cylinder 1 as described above, then a pressure surge will take place at the instant indicated by "A" on the left side in FIG. 6 because of the response delay of the normally open solenoid valve V.sub.3. Also, when the operation of the hydraulic cylinder 1 ends, all the solenoid valves are deenergized simultaneously and so a pressure surge is produced as indicated by "B" on the right side in FIG. 6 also because of the response delay of the normally closed solenoid valve V.sub.1.
As is well known in the art, such a pressure surge is created by transformation of the kinetic energy of oil into elastic energy and acts as an oil hammer. Accordingly, if the hydraulic pressure generator 2 is equipped with an accumulator 20 to store the produced hydraulic pressure, and if a motor is started and stopped with a high-pressure switch PSW2 and a low-pressure switch PSW1 to drive a pump, then these switches may malfunction.
In particular, the aforementioned pressure surge stimulates the pressure switches PSW1 and PSW2 to change into other state at a pressure lower than the pressure at which the switch PSW1 is to be switched to other state and at a pressure higher than the pressure at which the switch PSW2 is to be switched to other state, respectively. Therefore, the switches PSW1 and PSW2 are actuated much more frequently. This increases the frequency at which the motor for the pump is actuated. As a result, it is inevitable that the pressure switches PSW1, PSW2, and the motor for the pump age prematurely.